1. Field of the Invention
The present invention relates to an image pickup apparatus for capturing a video image.
2. Related Background Art
In recent years, it has become possible through the development of the digital signal processing technique that a large quantity of information including moving pictures, still pictures, or voices is digitally encoded with high efficiency to be recorded in a small magnetic recording medium or a small optical recording medium or to be transmitted to a communication medium. With the further development of such a technique, an image pickup apparatus has been developed which can readily capture a high quality image of an object to immediately output the resultant image signal to a communication medium.
Recently, an MPEG (Moving Picture Expert Group) encoding technique has been widely used for the encoding of a moving picture. FIG. 5 is an example of an image pickup apparatus using the MPEG encoding technique. An image signal of an image picked up by an image pickup unit 501 is supplied to an MPEG encoding unit 502. As for the MPEG encoding, an intraframe encoding method of encoding using a correlation within the same frame, and an interframe encoding method of encoding using a correlation between a preceding frame and a subsequent frame are used.
FIG. 6 is a diagram showing an example of a structure of encoded data output from the MPEG encoding unit 502. In FIG. 6, reference numerals 610 and 630 denote intraframe-encoded I pictures, respectively. In addition, reference numerals 611 to 621, and 631 to 641 denote interframe-encoded pictures, respectively, which contain P pictures predictive interframe-encoded in a forward direction, and B pictures predictive interframe-encoded in a bidirection. These pictures are collectively output in the form of picture groups 601 to 607 each having a predetermined number of pictures and classification. At least one I picture is contained in one picture group, and other interframe-encoded pictures are reproduced on the basis of information of the I picture.
An image signal encoded in such a manner is supplied to a recording unit 503 to be saved in an arbitrary recording medium 504. In addition, at the same time, the encoded image signal is supplied to a communication unit 505 to be transmitted to the outside of the image pickup apparatus through a stream output terminal 506. In such an image pickup apparatus, not only an image signal can be saved in a recording medium, but also the stream output terminal can be connected to a computer or a television so that the image pickup apparatus can be utilized for various kinds of applications such as an image delivery and a visual telephone.
A case is considered where in the image pickup apparatus shown in FIG. 5, an instruction to start recording an image signal onto the recording medium 504 is issued in the middle of transmission of an encoded image signal through the stream output terminal 506, e.g., an instruction to start a recording operation is issued at a timing T in FIG. 6, i.e., at a timing corresponding to a picture 615 within the picture group 603.
In this case, an image signal is recorded on the recording medium 504 from the interframe-encoded picture 615 in the middle of the picture group 603. In the picture groups shown in FIG. 6, such a structure is adopted that each of the picture groups contains one I picture at a head of the picture group. Thus, when the image signal is recorded in the middle of the picture group 603, the intraframe-encoded I picture 610 of the picture group 603 remains not recorded and is lacked. As a result, though the intraframe-encoded pictures 615 to 621 contained in the picture group 603 are properly recorded, these pictures 615 to 621 cannot be properly decoded.
As a result, in such a prior art example, a problem arises in that a picture group which can not be properly decoded is almost, surely generated right after start of each image capture, and hence during a reproduction time period of a head picture group, an image is caused to be frozen.
In order to avoid this problem, as in U.S. Pat. No. 5,774,624, such a technique was proposed that a picture group is newly generated in accordance with an instruction to start a recording operation, and no matter what timing the recording is started, an I picture is always included in a head picture group.
When the technique described in U.S. Pat. No. 5,774,624 is used, an image signal recorded on a recording medium can be properly reproduced from a head. However, as for the encoded data output from the MPEG encoding unit 502 to the stream output terminal, when an instruction to start a recording operation is issued in the middle of the picture group, the picture group of the stream output signal is divided into parts at this time point. As a result, a P picture required to reproduce a B picture, for example, may be lacked in some cases, and hence it is impossible to output an encoded image signal which can be reproduced without causing problems in both recording and transmission.